In the first place, there are at least three different kinds of RNA. The
largest quantity is a special kind called ribosomal RNA, or r-RNA. It is
found in close conjunction with proteins and makes up the structural
frame upon which the protein-synthesizing machinery is built. The r-RNA
and the proteins to which it is firmly bound form the ribosomes, the
RNA-rich microsomes that are attached to the endoplasmic reticulum.
Proteins are synthesized on ribosomes. We shall see later what
determines the differences among proteins and how these differences are
dictated directly by RNA and indirectly by DNA.
Besides r-RNA, there is a kind of RNA called soluble RNA, or transfer
RNA, or s-RNA. It combines with r-RNA to complete the sequence of events
that synthesizes the proteins. A bond between r-RNA and s-RNA is
established by a third RNA molecule called messenger RNA, or template
RNA, or m-RNA. This m-RNA molecule is truly the messenger that carries
the genetic message from DNA to the protein-synthesizing apparatus.
Dr. Michael Shimkin, a Temple University scientist, in his analogy has
compared the DNA → RNA → protein sequence to the activities of a
newspaper staff. DNA is the editor; m-RNA molecules are copyboys who
carry the editorials to the typesetters, the r-RNA and s-RNA, who then
take the “letters” of nucleic acid and set them into slots in accordance
with the editor’s directions. There are also workers who melt down
outworn letters and still other workers who make new letters for further
use; these are the enzymes, special kinds of proteins. If we wish to
continue the analogy, we may say that each kind of cell in the organism
has a different subeditor, who writes that cell’s own editorial.
Actually we might say that all cells have the same board of editors in
common, but only one editor functions in any given type of cell. In
biological terms this means that only a portion of all the cellular DNA
is active in each cell.
The active DNA is the DNA that makes m-RNA that will carry instructions
to the protein-synthesizing machinery of that type of cell. Cells of the
same organism therefore differ from each other on the basis of the
segment of DNA that is active in making m-RNA. Let us now see how we can
use radioactive isotopes to investigate the synthesis of RNA.